1. Field of the Invention
The invention relates to a device for the intermittent application of liquids, such as adhesive, in particular hot-melt adhesives comprising valve means for interrupting the flow of the liquid from a reservoir to a nozzle. The device according to the invention is particularly suitable for applying viscous material such as molten hot-melt adhesive.
2. Description of the Prior Art
Such devices are widely known. The valve means consists in many such known devices of a simple shutoff valve which is intermittently opened and closed. With such a construction problems arise in so far as upon opening the valve means, which to avoid large accumulations of liquid between valve and discharge opening should be disposed close to the discharge opening of the application nozzle, in the line between the reservoir and the valve the full pressure of the supply pump undiminished by dynamic pressure losses builds up.
When the valve is now suddenly opened, as is required with rapid applications with high interruption frequency, a relatively large amount of adhesive shoots out at the start of the application onto the material to be coated so that there the coating becomes undesirably thick. Also, with increasing relative speed between the material to be coated and application nozzle the start and the end of the coating becomes continuously less clean. To obviate the first difficulty the simple shutoff valve in the adhesive supply line has already been replaced by a valve means which operates by the principle of a three-way cock and in which in the application-free time the adhesive in the supply line is not stationary but flows back via a return line which is opened whenever the adhesive supply to the nozzle is closed. Admittedly, in this manner the constant change between full backpressure and dynamic pressure in the nozzle region is avoided. However, even with such a valve design the cleanness of the start of the application is in need of improvement, in particular at high frequencies of the application interruptions and high relative speed between web and application nozzle.